Dark mode code block ("CodeMirror") sections display a mysterious white square

### Description

When looking at the interactive code block (maybe called "CodeMirror") sections in dark mode, there is a white square that doesn't seem to do anything. I could see this on Chrome, Firefox, and Safari.

### Steps to Reproduce

1. Go to a page with an interactive code block (e.g., https://graphql.org/learn/queries/)
2. You may see something like this:

### Expected Result

I would expect not to see the white square.

### Actual Result

...I saw the white square.

### Additional Context

There's a small chunk of CSS that sets this white square to be white:
```css
.CodeMirror-scrollbar-filler,
.CodeMirror-gutter-filler {
background-color: white; /* The little square between H and V scrollbars */
}
```
On light mode, this makes the square invisible (because the background is also white). However, this CSS doesn't seem to account for dark mode.

My proposed fix is a little lazy. I'll just make the square invisible on dark mode:
```css
.CodeMirror-scrollbar-filler:is(html[class~=dark] *) ,
.CodeMirror-gutter-filler:is(html[class~=dark] *) {
background-color: transparent; /* The little square between H and V scrollbars */
}
```

I think that a larger fix might be ideal in the future, because the scrollbars are invisible anyway, so this square doesn't seem to have any use (see: #1617). But maybe my proposed fix is OK in the meantime.

Start on graphql.org/learn/queries/ and inspect the interactive CodeMirror section in dark mode, then locate the CSS rules for .CodeMirror-scrollbar-filler and .CodeMirror-gutter-filler. Done means the white square is no longer visible in dark mode while the interactive code block still works; check the result in Chrome, Firefox, and Safari.

Written by the indexing model from the issue text.